Combine soy sauce, water and cornstarch; set aside. In a largeskillet or wok over high heat, stir-fry half of the beef in 1tablespoon of oil until browned. Remove and repeat with remainingbeef, adding more oil as needed. Remove beef and keep warm. Addonions, celery and green pepper to pan; stir-fry until crisp-tender,about 3-4 minutes. Return beef to pan. Stir soy sauce mixtureand add to pan. Cook and stir for 2 minutes or until sauce isthickened and bubbly. Add tomatoes; heat through. Serve overrice.